The invention relates generally to ultracapacitors and, more particularly, to a system and method for performing ultracapacitor cell balancing.
Ultracapacitors, also known as supercapacitors, are electrochemical capacitors that have a high energy density as compared to common capacitors. Typically a capacitance of an ultracapacitor is on the order of thousands of times greater than a conventional electrolytic capacitor. In contrast to conventional capacitors, ultracapacitors typically use plates that are two layers of the same substrate that form an electrical double layer. The plates are typically separated by a nanoporous material such as activated charcoal that allows the separation to be in the nanometer range. Because of the very high surface area of the nanoporous material, many charge carriers can be stored in a given volume. The overall surface area of the nanoporous material is vastly greater than the plates of a conventional capacitor, hence the very large increase in capacitance compared to conventional capacitors.
The nanoporous material is susceptible to voltage breakdown and is thus limited, typically, to operating voltages in the range of 2-3 volts. Nominally, ultracapacitors may operate typically at 2.5 volts and during extremes may be taken to, for instance, 3.6 volts. However, such extremes are detrimental to the life of the ultracapacitor and failure may occur, which may result in an open cell. Further, the expected lifetime of an ultracapacitor is temperature dependent as well. Thus, for a given nominal operating condition of, for instance, 2.5 volts at a temperature of 25° C., the corresponding nominal life of the ultracapacitor is reduced at increased operating temperatures and/or operating voltages.
Large ultracapacitors, thus, may include designs having thousands of farads that are capable of 5-10 watt-hr/kg (Wh/kg) storage energy or more and on the order of thousands of watt/kg (W/kg) power density. As such, they are capable of providing high energy storage with quick discharge that makes them ideal for applications where quick power bursts and high energy storage are desired. Such applications may include but are not limited to regenerative braking systems, vehicle starting in cold conditions, crane lifting, and plug-in hybrid electric vehicles. Ultracapacitors are rechargeable many times over if operated within nominal conditions, they exhibit low self-discharge, and provide excellent stability over a range of temperatures.
Most such applications, however, use working voltages that are greater than the nominal 2.5 volts typically provided by an ultracapacitor. Thus, to obtain working voltages, ultracapacitors are typically connected in stacks or series of cells. However, due to temperature gradients and manufacturing irregularities, imbalance can occur between the cells which can lead to poor system operation or failure due to overvoltage of one or more of the ultracapacitors within the stack. Typically, under nominal operating conditions, an ultracapacitor can have 10 years and 1 million cycles or more. However, such life is negatively affected by operating at overvoltage, and actual life can be based on a prorated or percentage time spent operating at the elevated voltage or temperature. If voltage balancing is not provided, voltages become imbalanced, thus the entire stack is typically operated at a lower voltage in order to keep the worst case or “maverick” cell at or below nominal voltage. Further, fully failed cells cause an open of the cell, thus preventing all others within a string or sub-stack from delivering power, which drops energy storage as the square of the voltage, reducing storage and performance.
To avoid this, active cell balancing circuits and passive parallel resistance circuits have been used to prevent exceeding the maximum cell voltage in a given cell. Though these solutions may offer improvements for some systems, they introduce tradeoffs as well. For instance, passive resistance circuits may tend to increase self-discharge for many applications and may be used to periodically discharge or dump energy therefrom on an occasional basis. Active and individual cell balancing may be effective, but adds cost—both in complexity to the system design and complexity of operation.
Therefore, an apparatus and method of performing ultracapacitor cell balancing within a stack that overcomes the aforementioned drawbacks would be desirable.